As a relatively safe (compared to harder drugs) and easy-to-buy gateway drug, Marijuana is common in high schools and among teenagers. Marijuana use is increasing in high schools, especially now that many states are voting to legalize and regulate it, similar to alcohol and tobacco. According to drugabuse.gov:

In 2011, 7.2 percent of 8th graders, 17.6 percent of 10th graders, and 22.6 percent of 12th graders used marijuana in the past month, up from 5.7 percent, 14.2 percent, and 18.8 percent in 2007.

This rise may be due to the increasing acceptability of marijuana, with portrayals being more positive in media and culture. In the school system -- not the "official" venue, but the school culture -- drug use is more accepted as part of life, and even part of a specific lifestyle; in other words, going to high school means using a certain amount of drugs or alcohol. Peer pressure, as well as the disinterest of school officials -- who want to have peaceful schools, not conflict -- means that marijuana use is likely to continue increasing, especially if it becomes federally legalized. After all, although it is illegal for minors to consume tobacco and alcohol, the letter of the law does nothing to actually deter use. Similarly, the full illegality of marijuana does nothing to keep it out of the hands of minors; alternatively, legalization may be beneficial as dealers are forced to conform to the law, and may suffer punishment for dealing to minors.